1. Field of the Invention
This invention relates to a fuel feeding system and method for an internal combustion engine, especially for a lean-burn internal combustion engine which performs a lean-burn operation at an air/fuel ratio on a side leaner than a stoichiometric air/fuel ratio under predetermined operation conditions.
2. Description of the Related Art
Lean-burn internal combustion engines (i.e., so-called lean-burn engines) have been provided in recent years, which perform a lean-burn operation at an air/fuel ratio on a side leaner than a stoichiometric air/fuel ratio under predetermined operation conditions.
In such lean-burn engines, the air/fuel ratio is set as high as possible (in other words, an air-fuel mixture is set as lean as possible) during a lean-burn operation so that the emission of NOx can be reduced. The value of the air/fuel ratio is generally set close to a limit within which the air-fuel mixture can undergo stable combustion (i.e., a lean limit).
By performing such a lean-burn operation, it is possible to significantly improve the gas mileage while suppressing the emission of NOx.
To perform a lean-burn operation, it is the common practice to control the state of combustion by a controller. It has been reported in articles and the like to estimate an engine torque from an angular acceleration of a crankshaft in such control.
Further, Japanese Patent Application Laid-Open (Kokai) No. SHO 58-217732, for example, discloses the technique that, when fluctuations in the revolution speed of an engine are small, the state of combustion is determined to be stable and the air/fuel ratio is hence controlled to a lean side but, when such fluctuations are large, the state of combustion is determined to be unstable and the air/fuel ratio is controlled to a rich side.
In the above-described technique that an engine torque is estimated from an angular acceleration of a crankshaft, estimation and control of the state of an engine are however performed moment by moment on the basis of varying momentary values. It has not been contemplated to perform stable and precise control at predetermined intervals while taking into consideration the probabilistic and statistical nature of engine torque.
On the other hand, the following problem arises when control is performed to permit leaner combustion while detecting the stability/instability of combustion like the technique of Japanese Patent Application Laid-Open (Kokai) No. SHO 58-217732.
Namely, learning of the state of combustion is effected in a lean burn operation zone. Assume that the state of operation of an engine has again entered the lean burn zone subsequent to departure from the lean burn operation zone. Any attempt to learn variations among cylinders from the beginning leads to the problem that no lean-burn operation at a combustion limit can be achieved until this learning is completed.
In an operation zone where fuel is injected, for example, in a relatively large quantity or in contrast, in a relatively small quantity, linearity may not be maintained with respect to the fuel injection characteristic of an injector in some instances. In such a case, detected data lack accuracy so that the leaning of the state of combustion, namely, the learning of fluctuations in revolutions cannot be conducted correctly, thereby making it impossible to perform precise control of the air/fuel ratio for achieving combustion at the lean limit. This may lead to unstable combustion beyond a combustion limit in some instances.